The present invention relates to resistors used for voltage dividers. More particularly, a device and method is provided for dividing a voltage across integrated circuit resistors.
Electronic resistors are fabricated by using several different techniques. Fabrication methods include depositing a thin or thick resistive films on a substrate or forming a resistive path directly within a substrate, typically silicon, by using a dopant species. Such resistors are often used in microelectronics applications such as integrated circuits and hybrid microcircuits. Such resistors may be used in a resistor divider network that is a discrete circuit or may be used as part of a complicated integrated circuit. Resistive films often include polysilicon or nickel-chrome and dopant species often include boron, phosphorus or arsenic. Regardless of the type of resistor material or fabrication process, the resistance of such a microelectronic resistor can be generally described by Equation 1: ##EQU1## where R is the resistor value in Ohms, .OMEGA./.quadrature. is the sheet resistance in Ohms per square, L is the length of the resistor, and W is the width of the resistor. As known in the art, a square is dimensionless and is simply a portion of the resistor consisting of one unit length and one unit width.
Resistors may be used to create voltage dividers by providing a voltage across or current through the resistor and providing an output connection along some portion of the resistor. An individual voltage output of a microelectronic resistor voltage divider generally has a linear relationship with the voltage input of the resistor voltage divider. Furthermore, a voltage divider may have multiple outputs and each separate voltage output may have a linear relationship with the voltage input. On a voltage divider with multiple outputs, it is instructive to number each output consecutively starting from the top of the resistor divider progressing to the bottom of the resistor divider. When a plot is generated relating the input voltage attenuation of each output voltage to the respective output number, with one axis of the plot being the attenuation value and the other axis being the output number, this plot will be linear or nonlinear plot which is dependent upon the spacing of the outputs of the voltage divider.
Linear voltage dividers are defined as having a substantially linear plot of the voltage attenuation of each output and the related output numbers. Nonlinear voltage dividers are defined as having a substantially nonlinear plot of the voltage attenuation of each output and the related output numbers. The relationship between the outputs of the nonlinear voltage divider may be, for example, a logarithmic function, a square law function, or an exponential function. The individual transfer function between a voltage input and a specific output in a nonlinear voltage divider, though, may still be generally linear.
Voltage or current input and voltage output contact sites or taps may be made to a resistor in a variety of ways. FIG. 1 shows integrated circuit or hybrid microcircuit resistor 1 with taps 2 connecting from the top of the main body of resistor 1. In FIG. 1, taps 2 may simply consist of interconnect contacts, such as aluminum contacts. Alternatively, taps 2 may be upward extensions of the resistive film that are then connected to an interconnect lead. Other tap arrangements exist. For example, FIG. 2 shows resistor 20 having a main body 34 and output taps 22 and 24 which extend from main body 34 in order to provide a location for output contact sites 28 and 30. Input contact sites 26 and 32 are also provided. Many other geometric arrangements may be selected for taps. Furthermore, the tap spacing may be either linear or nonlinear along the resistor body. If all taps are spaced at substantially equal distance intervals along the resistor body, then linear spaced taps are created. Therefore, with linear spacing all resistor segments, whether between an input and an adjacent output or between adjacent outputs, are of equal length. If the taps are spaced such that all resistor segments are not of equal length than nonlinear tap spacing has been created.
Generally, contact sites, taps, and their placement will have some impact on each specific electrical transfer function of the voltage divider and, thus, will vary the actual transfer function from the transfer function of an ideal voltage divider. It is desirable to lessen the impact that output connections have on a transfer function of a resistor voltage divider. Furthermore, the impact of output connections is generally more severe in nonlinear voltage dividers than in linear voltage dividers. Therefore, it is desirable to lessen the impact of taps in nonlinear voltage dividers such as, for example, voltage dividers that allow logarithmic attenuation.
Microelectronic linear voltage dividers are generally made by providing linearly spaced output taps along the resistor body, while microelectronic nonlinear voltage dividers are generally made by providing nonlinear spacing of the output taps of a voltage divider. Alternatively, it is known in the art to provide a series of linearly spaced taps where all taps are accessible to the user, and allow a user to select the specific desired output taps. Thus, a user may use a voltage divider that has linearly spaced taps as a nonlinear voltage divider by only selecting outputs that have substantially nonlinearly related transfer functions. Furthermore, by providing only linear spaced outputs, a desired output voltage that requires nonlinear spacing of taps can only be approximated. Though such systems may lessen the impact of tap resistance, excess circuitry space may be consumed because these systems may allow the user to access every tap, and thus, every tap has some corresponding access and decoding interconnects and circuitry. It is, therefore, desirable to lessen the impact resulting from tap connections while limiting the amount of circuit space and complexity, and provide a very accurate representation of the desired nonlinearly related transfer functions.